1. Field of Invention
The present invention relates to electrically conductive paste used in the manufacture of a printed circuit board or the like and a method of forming a circuit formed by the electrically conductive paste.
2. Field of Invention
As a method of manufacturing a printed circuit board, a method is known in which, as shown in FIG. 4, so-called electrically conductive paste 10 in which an electrically conductive filler such as copper, aluminum, and carbon black is dispersed in a thermoplastic resin or a thermosetting resin is coated on an insulating substrate 11 along a predetermined circuit pattern, and this insulating substrate 11 is placed in a heating oven 12 and is heated at a predetermined temperature so as to cure the conductive paste 10.
According to this method, it is an essential requirement for the insulating substrate 11 to withstand the curing temperature of the conductive paste 10. For example, in the case of copper phenolic conductive paste which is representative as this type of conductive paste 10, it is the common practice to effect heating at 160xc2x0 C. for 20 minutes or thereabouts, and as the insulating substrate 11 it is necessary to use a material capable of withstanding such heating. For this reason, a resin whose softening point is lower than the aforementioned heating temperature, such as polyethylene or polypropylene, cannot be used as the material of the insulating substrate 11, and therefore there is a restriction in design. In addition, even the insulating substrate 11 formed of a resin other than polyethylene or polypropylene has some expansion or shrinkage due to heating, so that the situation is such that the deformation of the insulating substrate 11 is unavoidable.
Accordingly, it is practiced to prevent the deformation of the insulating substrate 11 by making the thermal stress of the insulating substrate 11 small by heating only the circuit pattern formed by the conductive paste 10 instead of heating the overall insulating substrate.
Specifically, a magnetic material together with a conductive filler is contained in the conductive paste 10, and a circuit pattern is formed on the insulating substrate 11 by using this conductive paste 10. A magnetic field is caused to act in the circuit pattern by a high-frequency induction coil or the like so as to cause the magnetic material in the conductive paste 10 to generate heat, and the resin is cured by this heat, thereby forming the circuit. According to this high-frequency induction heating, as for the insulating material 11, since only the portion coated with the conductive paste 10 is heated, it is possible to make the deformation of the insulating substrate remarkably small as compared with the aforementioned heating by the oven.
In addition, a high-frequency induction heating-type adhesive agent in which magnetic metal chopped fibers and metal powder are dispersed in a resin is disclosed in JP-A-6-35570 as the conductive paste 10 suitable for such electromagnetic induction.
However, in the case of the conventional high-frequency induction heating-type conductive paste, since the conductive filler and heating elements (magnetic material) are formed separately, the heating elements between themselves and the heating elements and the conductive filler are separated by the resin, so that the heat from the heating element is difficult to be transmitted. For this reason, it is necessary to adopt a measure such as making the output of the high-frequency induction coil large or making the heating time longer, so that the manufacturing cost increases.
In addition, in a case where the conductive filler and the heating elements are poorly dispersed in the conductive paste, unevenness of curing occurs in the conductive paste, and the quality of the circuit obtained is low.
The invention has been devised in view of the above-described circumstances, and its object is to provide electrically conductive paste whose heat generating efficiency in high-frequency induction heating is high, which permits curing with small energy, and which is free from the unevenness of curing, as well as a method of forming a circuit which is of high quality and has high reliability at low cost by using the conductive paste.
To attain the above object, electrically conductive paste in accordance with the invention is provided in that complexes each made up of an electrically conductive filler and a heating element adapted to generate heat on electromagnetic induction are compounded with a resin.
In addition, to attain a similar object, a method of forming a circuit in accordance with the invention is provided in that a circuit pattern formed by electrically conductive paste in which complexes each made up of an electrically conductive filler and a heating element adapted to generate heat on electromagnetic induction are compounded with a resin is cured by electromagnetic induction.